This proposal addresses a current knowledge gap: why women are 2 to 8-fold more susceptible to ACL injury than men. Many ACL injuries occur when landing on one foot from a jump or when running and cutting. In Years 01-03 we used an instrumented cadaver construct to identify the direction of the impulsive compound 3-D loading that causes the largest peak ACL strain: compression + flexion moment + internal tibial axial torque. We now seek funds to use this cadaver testing construct, complete with non-linear quadriceps tensile stiffness, to determine the effect of gender on peak ACL relative strain under the above compound impulsive loading scenario. Each knee will initially be positioned at an initial flexion angle of 15 degrees via pretensioned quadriceps, medial and lateral hamstring and gastrocnemius muscle-tendon equivalents that allow the knee to flex under the impulsive load. In AIM 1 we will test the hypothesis in 22 male and female knees from size-matched cadavers that gender does not affect the peak ACL relative strain due to a 2*BW impulsive force and the above compound impulsive loading. Pilot data suggest ACL strain is larger in female knees. In AIM 2, to further delineate the injury mechanism, we will use a repeated measures design and 22 size-matched knees to test the hypothesis that medially-dominant quadriceps and hamstring knee muscle forces significantly increase peak ACL relative strain compared to laterally-dominant muscle forces, especially in females. Pilot data suggest this to be the case. In AIM 3 we will use a repeated measures design to test the hypothesis in 10 knees that swapping out the female quadriceps muscle-equivalent with a stiffer male counterpart will significantly reduce the peak ACL strain in the female knee under the compound impulsive loading; in 10 male knees the corresponding effect will not achieve significance. Pilot data suggest this is indeed the case. Insights from this research will help address the urgent need to better understand the role of gender in the mechanism of ACL injury to improve both risk factor surveillance and prevention programs. PUBLIC HEALTH RELEVANCE: The goal of this research is to better understand the mechanisms causing ACL injuries to be more common in women than men. Given the smaller cross-sectional area of the female ACL, in AIM 1 we will use fully instrumented cadaver knees to test the hypothesis that the peak ACL strain during a simulated jump landing is larger in females than males of the same size. In AIM 2 we will test the hypothesis that this strain is elevated in the female when the muscle forces crossing the knee on the inside and outside of the leg are not balanced. In AIM 3 we will test the hypothesis that increasing the tensile stiffness of the female quadriceps muscle significantly reduces peak ACL strain. Insights from this research should be useful for improving the efficacy of prevention programs designed to reduce the risk of ACL injury, particularly in females.